A polite act shows respect. But a new study of a common etiquette — holding a door for someone — suggests that courtesy may have a more practical, though unconscious, shared motivation: to reduce the work for those involved.

The research, by Joseph P Santamaria and David A Rosenbaum of Pennsylvania State University, is the first to combine two fields of study ordinarily considered unrelated: altruism and motor control. It was published in a 2011 issue of Psychological Science, a journal of the Association for Psychological Science.

“The way etiquette has been viewed by Emily Post — that you’re being proper by following social codes — is undoubtedly part of it,” said psychology professor Rosenbaum.

“Our insight is there is another contributor: the mental representation of other people’s physical effort. Substantial research in the field of motor control shows that people are good at estimating how much effort they and others expend,” Rosenbaum continued. “We realized that this concept could be extended to a shared-effort model of politeness.”

The researchers videotaped people approaching and passing through the door of a university building. The tapes were analyzed for the relationships among several behaviors: Did the first person hold the door for a follower or followers and for how long? How did the likelihood of holding the door depend on the distance between the first person at the door and whomever followed?

“The most important result,” Rosenbaum said, “was that when someone reached the door and two people followed, the first person at the door held the door longer than if only one person followed. The internal calculation on the part of the first arriver was, ‘My altruism will benefit more people, so I’ll hold the door longer.'”

Another finding: the followers who noticed the door-holder hastened their steps, helping to “fulfill the implicit pact” between themselves and the opener “to keep their joint effort below the sum of their individual door-opening efforts,” the authors write.

A more common explanation of why we extend a physical gesture of courtesy is what the researchers term the “critical distance” model: we do something for someone if she is simply near enough.

But the researchers found that model insufficient. “We need a way of describing why there is a change of probability” both of doing the task and of expending more time at it, said Rosenbaum. Is the critical distance 10 feet? Why not 50 feet? What is “near enough?” And why wait longer if more people are following?

“You still come back to the question of what the individuals are trying to achieve.”

Rosenbaum sees the shared-effort model as enhancing, not detracting from, our appreciation of good manners: “Here are people who will probably never see each other again,” he says, “but in this fleeting interaction, they reduce each others’ effort. This small gesture is uplifting for society.”